In the related art, a spark plug performing a firing of mixed gas by spark discharge (simply referred to as ‘discharge’) is used for an igniting plug of an engine, for example, an internal combustion engine of automobiles. In recent years, a plasma jet ignition plug in which the expansion of combustion is speedy and which can be surely fired for lean mix gas having high igniting limit fuel efficiency has been used as an ignition plug since a high output and low fuel efficiency of an internal combustion engine are required.
In such a plasma jet ignition plug, a spark discharge gap between a center electrode and a ground electrode is formed when connected to an electric source. A plasma jet ignition plug surrounds a vicinity of this spark ignition gap using an insulator such as ceramics, and thus has a structure forming a discharge space of small volume called a cavity. A plasma jet ignition plug using an overlap type electric source is described with reference to one example (e.g., see patent document 1). In an ignition of mixed gas, first, a high voltage is applied between a center electrode and a ground electrode, and then a spark discharge (also called ‘trigger discharge) is produced. Due to dielectric breakdown produced at the time of spark discharge, a current can flows between both electrodes through a relatively low voltage. Thus, more energy is provided, thereby transiting a discharge status, and therefore plasma is formed in the cavity. Further, the formed plasma is extruded through a communicating hole, so that an ignition of nixed gas is performed. One effusion of plasma corresponds to each stroke.
In such a plasma jet ignition plug, a higher current than typically used to spark discharge in a general spark plug is required to flow through the spark discharge gap when plasma is formed. In order to increase a flowing current, it is necessary to lower the electric resistance value of the circuit through which the current flows, and normally a resistor is no provided in the circuit of an ignition apparatus or inside the plasma jet ignition plug (e.g., see patent document 2).